Assesing the quality of water along the course of the River Lyn

One method that geographers can use to evaluate the quality of water within a river system such as the Lyn, is to sample for the presence or not of a range of indicator species. All species have specific requirements of the habitat in which they attempt to survive. The requirements of some species are much more specific and narrowly defined than for othes. For example mayfly nymphs can usually only survive in water that is relatively unpolluted and has a high degree of dissolved oxygen (such as a healthy upland stream). However other species such as some worms and midge larvae are far more tolerant of low water quality and can survive in almost any water body regardless of water quality.

The Biological Monitoring Working Party (BMWP)
Score is an method to measure the biological quality of rivers using invertebrates
as indicators. This score is widely used by organisations such as the
Environment Agency to monitor water quality in rivers..

Sites are sampled, usually by "pebble washing" or by kick/sweep with a standard pond net. . Any invertebrates caught are
identified to family level. Each family is then given a score between 1 and 10. The index does not take into account the abundance
of each family, simply the presence or absence of each family at the site.

The score each family gets reflects their tolerance
of pollution (different aquatic invertebrates have different tolerances to
pollutants). For example stonefly, and some mayfly families, are not very
tolerant of pollution, so these families are given a score of 10. The presence
of these high-scoring invertebrate families indicates a site with unpolluted
water. The lowest scoring invertebrates are worms which score 1, as they are much
more tolerant of pollution. They can tolerate heavily polluted waterways but are
still found in unpolluted conditions as well.

Consolidate your thinking

Watch the video below (the key section for this enquiry begins at about 3'10 into the film) to familiarise yourself with the approach to water sampling required to get the data you need from both pebble washing and kick sampling.

There are a range of easily accessible water sampling sites along the river Lyn that you can collect data from. These can be seen on this map.

There is also a link here to a recording sheet on which you can summarise the data you have collected and then derive a total score that in turn will allow you reach a conclusion regarding the quality of the water you have sampled.